I got a pair of TAD (Tube Amp Doctor) Cryo'd KT88s with an amp I bought. They are listed in the Rachael manual as an approved tube, but I read somewhere that Steve felt they might sound thin due to higher bias.

At first, I agreed with him, but I gave them some time and they now have a fuller sound and better controlled bass with more slam. They are currently my preferred power tube in combination with RCA NOS 5U4G rectifier and Gold Lion 6922s.

I also have some KT88s (Genalex) that I got with another amp. I have used them in my Rachael for a day or so just to make sure that they work as a potential back up. For now, I am sticking with EL34s just because I want to.

Interesting, I would like to try KT88 and see. I am picking up some Tesla EL34 now, and some Gold Lion re-issue KT88 might be a good choice!

BTW - what are your setups and do you have any hum? I have a straight amp and it is very quiet overall (no spurious noises etc), but I'm trouble shooting some power issues in the house and do have some low-level hum.

I think the biggest difference is in the bass. Much tighter and more powerful with the KT88s. The mids/highs are smoother than the EL34s I have (two sets) .Perhaps a bit rolled off, but it works well with my full range drivers.

I ran a more controlled test. The KT88s were in fact not as loud (around 2db lower) as my JJ EL34s and Tung Sol EL34Bs.

But, the KT88s sounded fuller. More body. They just seemed to fill the room more and were not as forward. I used a 100hz test tone for all the tests so wasn't a different frequency impacting the results. Not sure I understand this.

While keeping the input/output tubes the same (6922s/EL34Bs) I switched out rectifiers. None impacted the volume.

So while I don't know what I'm doing really, I think I have shown that KT88s produce a different sound, but not a louder sound. Also, I thought that at least one of my rectifiers made for louder sound. My NOS RCA. 5u4g, TAD GZ34 and Ruby 5u4G all played the same volume.

I don't know much about the topic either, but it seems to me that narrowing your reference tone to only 100 Hz is not representative of the real frequency distribution typically exposed in normal music listening.

The result would be constricted to the 100 Hz realm only, thus not a reference you could extrapolate to the rest of the distribution spectrum. My $0.02

Thanks. I agree with your conclusion. It was more or less a quick test. KT88 seem to do bass better so I chose a middle of the road bass frequency. I was actually surprised that it was not louder than my other tubes given my perception.

I could repeat the test with pink noise or maybe play an entire song and record the average SPL.

"In general, the KT88 tube has a magical mid-range similar to the smaller EL34 tube, but with low and high end extremes that reach much further."

And this from an audiogon post comparing EL34 and KT88:

On sound quality, the consensus seems to be that the '34 has more lushness or richness in the midrange than almost any other mid- to hi-power output tube while being a little soft at the frequency extremes. The '88 (and its very close cousin the 6550) is more balanced across the frequency band, exhibiting more-extended and -controlled bass and better-quality treble than the '34 while lacking the '34's (euphonic?) midrange richness. These differences probably can be swamped by the extremes of different brands of the 2; the above is average or typical.

There seems to be a price to pay for the traditional lushness in the EL34's midrange through rolled ends on the frequency range. Linearity is supposed to be more desirable but that midrange 'to die for' has become an addiction for many a listener.

Linearity means, in this case, no overemphasis in a particular frequency subgroup ... That is why the lower frequencies and higher ones seem fuller and heavier, but I bet the midrange is not to die for anymore ... tradeoffs?